Helicopter swashplate control comprises the control of the orientation of the swashplate for the main rotor. This swashplate has one degree of translational freedom, referred to as "collective", and two degrees of rotational freedom, referred to as "roll" and "pitch". Generally, control of the swashplate orientation requires the specification of the orientation of a plane in space. Since three non-colinear points specify a plane, the conventional method of swashplate control has employed three non-colinear hydraulic actuators which are controlled independently to achieve the positions that define the desired orientation of the swashplate.
Redundant actuation systems have been developed for the control of helicopter swashplates. The redundancy is employed to improve reliability and survivability. In the prior art systems redundancy has been introduced either by employing more than one hydraulic ram piston to drive each of the three control shafts (force-summing), or by using more than one control valve to regulate a single ram piston driving each control shaft (redundant secondary stage). Both of these techniques concentrate all of the actuating hardware at three positions on the swashplate, thereby limiting the ability of the swashplate controller to survive physical damage and certain hardware malfunctions, such as may be caused by fatigue, failure of the control shaft, or a seized ram piston.
A somewhat different mode of swashplate control has recently been suggested to circumvent these apparent disadvantages, see Corlock, G., et al., "STAR Flight Control System", presented at the Helicopter Flight Control Meeting of the American Helicopter Society, October 1978. This system consists of five actuators coupled symmetrically around the swashplate to form a "star" pattern, thereby allowing for swashplate orientation control by the remaining actuators in the presence of as many as two inoperative actuators. The suggested control law by-passes all but three actuators at all times. A selection logic scheme detects inoperative actuators, which are subsequently by-passed at all times. The controller commands the remaining three actuators to achieve the desired positions, in a manner similar to a conventional controller.
Which this system overcomes many of the disadvantages of the previous systems, it is possible to have substantial asymmetry in the loading pattern of the three controlled actuators under certain conditions (for example, when two adjacent actuators have failed). Under such conditions, relatively high loading is present for individual actuators, which tends to increase the possibility of additional failures. Furthermore, the non-uniformity of the actuator control system precludes the use of powerful comparison testing strategies for fault detection and makes necessary the use of less reliable self-test techniques for this purpose. In addition, the by-passed actuators may be subject to undesirable start-up transients when they are used to replace one of the originally selected active actuators in the control logic.
Accordingly, it is an object of the present invention to provide an improved redundant helicopter swashplate control system.
It is another object of the present system to provide an improved helicopter swashplate control system which optimally distributes the load among component actuators.
It is yet another object of the present invention to provide an improved helicopter swashplate control system minimizing actuator transients.